Update md and website to replace Jira by GitHub Issues

[jbonofre]

The website still mentions Jira (https://activemq.apache.org/issues) whereas we now use GitHub Issues.

It's worth to do a complete pass.

[sergio-d-lemos]

On it. Checking the documentation (activemq-website) for where we mention Jira

[jbonofre]

@sergio-d-lemos Thanks! I think I saw old DockerHub repository too (https://hub.docker.com/r/apache/activemq-classic instead of https://hub.docker.com/r/apache/activemq). If you see old DockerHub repository, can you also please update to the new/correct one?

[jbonofre]

@sergio-d-lemos I assigned this ticket to you.